An important problem in the biology of mammalian cell growth concerns the regulation of messenger RNA function. As an example of contrasting states of growth, we have studied contact-inhibited fibroblast Balb C/3T3 cells and compared the properties of their messenger RNA (mRNA) with those from serum induced proliferating cells. Our studies show that the transition from resting to proliferating state is accompanied by a marked stimulation of translational initiation, and a greater stability of poly (A) containing RNA molecules. In order to gain further understanding of the regulation of mRNA metabolism, our aim in continuing these studies is to elucidate properties of mRNA that are relevant to its functions. Since the messenger molecules are associated with proteins through most of their life history in the cell, we sought to examine the biology of messenger molecules at the ribonucleoprotein level of organization. The location of the principal mRNA binding proteins will be determined by sequence analysis of the protein protected regions in molecules labeled in vitro at 5' termini after removal of "cap" structure, and labeling with (32p)ATP and T4 polynucleotide Kinase. Cell growth specific differences in mRNA "cap" structures will be determined by similar in vitro labeling of 5'-termini and resolution of the ribonuclease T2 resistance oligonucleotides. These studies will aid our understanding of the biology of mRNA function in contact inhibited mammalian cell growth regulation in general.